This patch solves build issues with -Werror=implicit-function-declaration
enabled.

- safe_flock is a function from flocklnx.c but header file for consumers
  is missing, the simplest was to add function prototype to other
  header file.
- utime.h is needed also in multiple places but os_slx.h header file is
  used in all of them

Bug: https://bugs.gentoo.org/870478
--- a/src/osdep/unix/os_lnx.h
+++ b/src/osdep/unix/os_lnx.h
@@ -46,6 +46,7 @@
 #include <sys/types.h>
 #include <dirent.h>
 #include <time.h>		/* for struct tm */
+#include <utime.h>
 #include <fcntl.h>
 #include <syslog.h>
 #include <sys/file.h>
@@ -57,6 +58,7 @@
 
 #define direct dirent
 
+int safe_flock(int, int);
 #define flock safe_flock
 
 
--- a/src/osdep/unix/os_slx.h
+++ b/src/osdep/unix/os_slx.h
@@ -46,6 +46,7 @@
 #include <sys/types.h>
 #include <dirent.h>
 #include <time.h>		/* for struct tm */
+#include <utime.h>
 #include <fcntl.h>
 #include <syslog.h>
 #include <sys/file.h>
@@ -57,6 +58,7 @@
 
 #define direct dirent
 
+int safe_flock(int, int);
 #define flock safe_flock